The present invention relates to a silicone-based antifoam composition or, more particularly, to a silicone-based antifoam composition having excellent stability against dilution and durability of the antifoam activity so as to be useful for foaming suppression in the industrial dyeing process, especially, at high temperatures without causing troubles of uneven dyeing.
Various types of antifoam compositions are known in the prior art and used for suppressing foaming and destroying foams, among which silicone-based antifoam compositions containing a dimethylpolysiloxane as the principal ingredient are most widely used in various processes in which suppression of foaming is of essential importance including various chemical industries, dyeing industries and the like by virtue of their excellent performance as compared with antifoam compositions of other types.
Since dimethylpolysiloxanes are inherently hydrophobic and hardly dispersible as such in water, silicone-based antifoam compositions are used usually in the form of an aqueous emulsion prepared by emulsifying a dimethylpolysiloxane in water using a surface active agent. A problem in such an emulsion-type silicone-based antifoam composition is that the emulsion is relatively unstable and sometimes destroyed in use especially when the foaming bath to be subjected to an antifoam treatment is at a high temperature or under a strong shearing force, for example, with vigorous agitation resulting in complete loss of the desired antifoam effect or rather in eventual promotion of foaming of the bath. The emulsions are also not quite stable in storage so that the dimethylpolysiloxane may be separated from the emulsion during prolonged storage so that use of such a degraded emulsion-type silicone-based antifoam composition in the dyeing process sometimes causes a trouble of uneven dyeing or occurrence of oil spots on the dyed products due to the water-repellency of the separated dimethylpolysiloxane.
Various attempts and proposals, of course, have been made to solve the above mentioned problems in silicone-based antifoam compositions. For example, U.S. Pat. No. 3,233,986 proposes use of an organopolysiloxane modified with polyoxyalkylene groups as the principal ingredient. An antifoam composition prepared by emulsifying such a polyoxyalkylene-modified organopolysiloxane in water, however, is not suitable for practical use owing to the relatively low effectiveness for destroying of foams and suppression of foaming. It is reported in Japanese Patent Publications 52-19836, 52-22638 and 55-23084 that the antifoam activity of an antifoam composition based on such a hydrophilic polyoxyalkylene-modified dimethylpolysiloxane can be improved when this base ingredient is combined with an oil compound composed of an unmodified organopolysiloxane, e.g., dimethylpolysiloxane, and a finely divided silica powder. Further, Japanese Patent Publication 58-58126 proposes use of an organopolysiloxane co-modified with long-chain alkyl groups and polyoxyalkylene groups as the base ingredient of an antifoam composition.
These silicone-based antifoam compositions after the above mentioned proposal for improvement are still unsatisfactory when they are used in the modern dyeing industry in which the dyeing bath is at a high temperature and sometimes under very strong mechanical shearing forces to destroy the emulsion of the organopolysiloxane which is coagulated into oil drops so as to cause serious troubles of uneven dyeing or stain of the dyed material with oil spots.
In view of the above mentioned problems relative to the silicone-based antifoam compositions used in industrial dyeing processes conducted, usually, at high temperatures, a further proposal has been made for preventing coagulation of the organopolysiloxane in an aqueous emulsion at high temperatures, according to which the antifoam composition is prepared from a polyoxyalkylene-modified organopolysiloxane of which the molar fraction of the oxyethylene units relative to the oxypropylene units is increased in the overall oxyalkylene units. The use of such a polyoxyalkylene-modified organopolysiloxane of an increased molar fraction of the oxyethylene units, however, is accompanied by several disadvantages that the dispersibility of the oil compound composed of a dimethylpolysiloxane and a finely divided silica powder is decreased and the stability of the emulsion is adversely affected by dilution, under mechanical shearing forces or at high temperatures so that such a silicone-based antifoam composition is not always suitable for practical use.